1. Field of the Invention
A weighing balance of the electromagnetic load compensation type includes a thermal conductor for dissipating the internal heat generated by the compensation coil to a temperature sensor located remotely from the air gap containing the compensation coil, thereby to modify the compensation current to produce a more accurate load measurement.
2. Brief Description of the Prior Art
Weighing scales or balances of the electromagnetic load compensation type are well known in the patented prior art, as evidenced, for example, by the U.S. Pat. Nos. to Kunz Nos. 4,545,448, 4,722,409 and 4,938,300, and Strickler No. 5,141,066, among others. In these known weighing systems, the magnitude of the load applied to the load-receiving member is displayed as a function of the compensation current supplied to the compensation coil to maintain the movable load-receiving member in a given position relative to its fixed support. The compensation coil is connected for movement with the load-receiving member and extends within the air gap of a stationary permanent magnetic field produced by a permanent magnet carried by the fixed support. The intensity of the magnetic field produced by the permanent magnet means is influenced, for example, by the load-responsive compensation current and/or by the heat generated by the compensation current in the coil, and consequently, compensation is required to eliminate the adverse influences affecting the accuracy of the load measurement. In this regard, one must distinguish between short-term and long-term temperature changes.
The compensation of long-term temperature changes, such as occur when the balance is turned on, has been known for a long time, and various proposals have been presented in the prior art.
In the electrical balance proposed in the German patent No. DE-GM-84118503, an aluminum tubular member is inserted by a press fit or by means of a heat-conducting filler between the active magnetic core and the magnetic yoke member. The aluminum tube contains a bore in which is mounted a temperature sensor that receives temperature changes produced by the heat dissipated from the pole piece with which the aluminum tube is in good heat-conducting engagement. The heat-conducting aluminum tube affords rapid heat dissipation from the pole piece of the permanent magnet system. It thus constitutes a heat diverting bridge that produces a reduced heating of the active magnetic core, and thus diminishes sensitivity changes in the permanent magnet system that are caused by heat. Regardless of the good heat conductivity of the aluminum tube, a temperature change may be sensed by the temperature sensor only after a certain degree of delay, which means that any electronic correction will also be delayed. The aluminum tube can also be arranged to form a gap relative to the active magnet core, and the heat from the pole flange can be dissipated directly downwardly for measurement by the temperature sensor. In both embodiments, the temperature change of the pole piece is measured indirectly, and the pole piece conducts further downwardly the radiation heat absorbed by the coil.
In the German patent No. DE-A1-3324402, a temperature sensor is inserted within a bore contained in the permanent magnet system to measure the heating of the magnet by the load-dependent current in the coil. A time delay is therefore unavoidable here also, since first of all, the pole piece and the permanent system have to be heated up before the temperature sensor can detect the temperature change. The deleterious consequences of the temperature change occurring in the permanent magnet appear immediately, and the corresponding compensation can occur only after some time delay.
The Japanese patent No. A-07218323 discloses an electromagnetic balance including a temperature sensor inserted within a recess contained in the pole piece directly opposite the coil for measuring the adjacent temperature in the magnetic system. A plate is provided that spans the recess. Using this known arrangement of the temperature sensor on the face of the pole piece, the temperature is measured before the dissipated heat of the coil can heat the pole flange and, more particularly, before the heat reaches the permanent magnet. This can result in an over reaction during the compensation of the effect of the temperature change on the permanent magnet. The temperature is measured directly within the effective range of the compensation coil, so that the temperature sensor is not subjected to heat in the same manner as the heat-sensitive permanent magnet from which the temperature change is produced. As distinguished from the previously discussed patents, the apparatus of the Japanese patent measures the temperature change too quickly, with the result that the measured temperature change is not in agreement with the change produced by the heating of the permanent magnet.